Electric sound recorder



Sept. 26, 1933. H, CLARK 1,928,169

ELECTRI C SOUND RECORDER Filed July 16, 1931 2 Sheets-Sheet l Sept. 26,1933. H; A. M. CLARK 1,928,169

ELECTRI C SOUND KECORDEE Filed July 16, 1931 2 Sheets-Sheet 2 I I I 78 mI i I U I nwe/vTaR BYZYHWW Patented Sept. 26,1933" I v -EDYSTAT S one];i j

ELECTRIC soUNp anooana'a Henry Arthur -Maish Clark,London, England,

assigndr to Columbia-Graphophone Limited, London, England 7 "ApplicationJuly 16, 1931, Serial No. 551,234,

Company,

a and in Great'Britain July 19, 1930.v

, 2 Claims. (01. 179-10041) This invention relates to moving coilapparatus such as sound recorders and. sound reproduce rs and likeapparatus with particular reference to the field systems adopted withsuch devices.

It is found in practice that in order to obtain sufiicient electromechanical damping in the moving .coilcircuit it is essential to use'a,mag

netic field of ten thousand or more lines per;

moving coil while maintainingthe weight of the apparatus at a minimumand with this object in view the invention consists in electromagneticmoving coil vibratory apparatus comprising a field system formed of oneor more magnetizing windings mounted-upon a pole or poles formedpartially or wholly of material having .a magnetic saturation fluxdensity-higher thanthat of pureiron.-

The invention further consists in .,electromagnetic moving coilvibratory apparatus comprising a field system formed of oneor moremagnetizing windings mounted upon a pole. or poles formedpartially orwholly of material such that theratio of itsmagnetic saturation fluxdensity to its specific gravity is greater thanthe same ratio for pureiron. 7 j The invention further consists in electromagnetic apparatus asset forth above comprising a "substantially U-shaped magnet formed oftwo poles connected together at one end by a uniting yoke, which maybeintegral with the poles.

The invention also consists. in electromagnetic apparatus as set forthabove wherein the field system is freely mounted (for example byfloating' upon rigid knife edges).

Further features of the invention will'be ap parent from the descriptiongiven hereafter of one modification thereof which will be more readilyunderstood by reference to'the accom panying drawings wherein: r

Figure 1 represents in sectional plan and Figure 2 represents insectional elevation along a the line 22 of Figure 1 the field "system ofa the system. r

' moving coil sound recorder' in accordance with the'invention.

Figure -3 represents a side view of'the recordermounted readyfor use inits bearings in a bracket,

partially broken away, and

Figure 4 represents an end view of the clampingplate and bracket'whi'c'hcarry the recorder.

In carrying my invention into effect in one convenient manner as, forexample, in its application to the moving coil sound-recording apparatusshown in Figures 1 and 2, I form the main .magnetic circuitconsisting ofthe poles a, a and uniting yoke b from a suitable cobalt iron alloyhaving a very high magnetic saturation value. For thispurpose-the alloydescribed by Yensen in the Journal-of the A. I.v E. E. for 1915 is verysuitable, comprising approximately 33, per cent. of cobalt and 67 percent. iron, corresponding to the chemical formula FezCo. The specificgravity of this alloy is not very different from that of iron so thatsince its magnetic saturation value is considerably greater than that ofiron the ratio of its saturation value to its specific grave ity isalsogreater than that' for ironand for any 1 given weight an increased fluxmay be secured or alternatively any desired flux may be obtained withless weight of this alloy than if ir'on were employed. The poles may begradually tapered or narrowed from the yoke towards their outer endssoas to maintain the flux density in'the system as nearly constant aspossiblev having regard to thelleakages that occur at different parts of.At the outerends of the poles a, a we provide .two pole-pieces lc cbetween which the moving coil (1 is located and which are tapered or, r

narrowed from the part where they join the poles towards'their outerends where they are in close proximity to the moving coiland the widthof the pole-pieces at their ends should. be very little more than thewidth of the coil sides in order to crowd the flux through the coil. Onthe other hand the height of the pole-pieces should be slightly greaterthan the height of the coil sides in order that the coil may be made toembrace a proportion of what would otherwise be a leakage flux and thusa slightly increased efficiency andwhich is shaped so as to providemaximum area across the path of the steady flux flowing across, themoving coil sides while at the same and allowing the central shaft,f.(if any) to pass :through it.

- In'the case of a transformer coupled single turn moving coil the coremust also be'designed with due consideration for the alternating fluxwhich travels in a direction normalize, the steady flux and hence amaterial must be chosen of composition and form such that the hysteresisand eddy current losses are a minimum. Moreover is preferable .toarrange that the core ejshall be. the first portionof the circuit tob'saturated. every other part of the iron circuitlbeing satr "tainers kand firmly grips them. At the outer "ends of'thi's plate are providedbearing plates 1n urated simultaneously netizingforce.

at axslightly' higher mag.-

The conditions above referred to make it preferable to employ alaminated coree (Fig. 2)

., being suitable) since while the cobaltalloy above j within the movingcoiland we may therefore conveniently form the same from pure iron sheet(iron soldunder the proprietary nameof Armco referred to would besuitable from the point of view of flux density it isnot so convenientto prepare in'thefform of'laminations; It must be understoodhoweverthatthe' use of laminations of the :alloy. above referred toor or anyother suitable alloy for'th'e core within-the coil is not precluded fromthe invention. I

I The magnetizing windings g contribute a large .pr'oportionof the total'weight of "the device and in order to extract the utmost efiiciencyfrom the windings so as'to be able to'r educe the weight I as much'aspossible'they are preferably graded.

jIh'e bobbins i (Fig. 1) may=be made of aluminium and are made to fitclosely on the tapered poles, and the windings are divided into, say;two por= tions', the inner part 91 being wound with a slightly smallergauge or" wire than' the outer part g2 while since'one effect dueto'leakage is that the exciting turns nearer the pole-pieces"are moreuseful" than those nearer the yoke I provide as many turns aspossible'close'to the ends'o'f the poles "as. for "example, I mayarrange-at the pole endsconical or tapered coils h' inseries with themain coils and 'shaped so as to house the maximum numberof turns withoutfouling the alternating my. iron-circuit on the core 6 above referredto. Moreover, the coils g, h'jare preferably short and"stumpy ratherthan long and slender'and all thejcoils are preferably enclosed in.spunaluminiumcontainers' 1c (Fig.1) 'so'that' they are thus surroundedby a'hea'vily' 'sh'ort-- circuited turnwhich prevents the rise ofexcessive ,back E. M..F.s when the fieldcurrent is suddenly switched oifand the use ot shunt; resistancesto absorb this EQM. F. is thus avoided1 Knife edges, o, on the inner surface of the bracketmeansifmicrophones, loud speakers or other What is claimed, 7 V

apparatus for cutting'a sound record in wax, com- ;such a manner as toaccommodate irregularities being utilized for'the purpose.

aflhe mounting of a recorder constructed in accordance withtheinvention, as described above, is effected by means of a clamping plateI (Figure 1) which at. a suitable point surrounds the conhaving V-shapednotches, and the whole device is slipped within a bracket n (Figure 3)of, inverted U-shape,rigidly secured to a fixed support and adapted to'overhang the recording table.

n receive the V-shaped notches of the plates m,

of contact. I

The invention is applicable to soundrecorders (either of the typeadapted to cut a sinuous record groove in wax or like material; orof-the type in which the moving coil-causes oscillations the recorderbeing thus pivoted about their point of amirror or like member in anoptical system to berecorded on a' film or by other light'sensitlvesound reproducers} or like moving coil apparatus, and is not to belimited to-the details of construction hereinbefore given by way ofexample since I may modify the construction and" arrangement of themoving coil, the number and mode-of disposition of the magnetic coilsemployed for the magnetic circuit asso ciated with the" moving coil andthe form, constructionand material of such magnetic circuit dependingupon any practical" requirements that mayhave to be fulfilled. V

1.- An electromagnetic moving coil recording prising a magneticfieldsystem formed of ma- 12o terial having a magnetic saturation-fluxdensity greater than thatof pure iron, a holder for said "system. andpivotal supports associated with said holder whereby the system isfreely mounted in in'the surface.

ZQAHY electromagnetic "moving coil recording apparatus according toclaim 1 wherein said magnetic fieldsy ste'm is formed of an alloycomprisiiigfiii per cent. cobalt and 6'7 per cent. iron.

' AR'I'HUR"MAISH CLARK.

